Functonalized cellular elastomer foam, and a use of a cellular elastomer foam as a catalyst substrate--

ABSTRACT

A method for modifying a cellular polymer foam with apparent porosity, which includes providing a cellular polymer foam with apparent porosity, placing the cellular polymer foam in contact with at least one compound in order to obtain a cellular polymer foam including on the surface thereof an intermediate phase formed from the compound having at least one catechol unit. The foam may be used as a catalyst substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Application of PCTInternational Application No. PCT/FR2015/051903 (filed on Jul. 9, 2015),under 35 U.S.C. § 371, which claims priority to French PatentApplication No. 1457055 (filed on Jul. 22, 2014), which are each herebyincorporated by reference in their respective entireties.

TECHNICAL FIELD

The invention relates to the field of porous solid materials, moreparticularly cellular polymer foams. It relates to a method formodifying the surface of elastomer cellular foams, in particular foamswith apparent porosity, so that they may be used as catalyst substrates.

BACKGROUND

Many catalysts are known whereof the catalytic phase is present in aporous solid material, and more particularly adsorbed to the surface ofa ceramic or metal cellular foam. For example, the most widely usedmethod for producing ceramic foams consists of impregnating a polymerfoam, most often a polyurethane or polyester foam, cut to the desiredgeometry, with a suspension of ceramic particles in an aqueous ororganic solvent. The excess suspension is discharged from the polymerfoam through the repeated application of compression or bycentrifugation, so as to keep only a fine layer of suspension on thestrands of the polymer. After one or several impregnations of thepolymer foam, the latter is dried so as to evacuate the solvent whilepreserving the mechanical integrity of the deposited layer of ceramicpowder. The foam is next sintered in order to obtain an inorganic foamusable as a catalyst substrate. This fairly complex manufacturing methodcreates a relatively high manufacturing cost.

One of the advantages of ceramic foams is their chemical and thermalstrength. However, excellent thermal strength is not always needed.Furthermore, ceramic foams have the drawback of concealing micro-cracksand other microstructural flaws that considerably decrease theirmechanical properties. Furthermore, in many cases, recovering the activemetal phase (catalyst) requires many chemical treatments.

SUMMARY

One aim of the present invention is to provide new catalyst substratesthat are an alternative to the catalyst substrates made from metal orceramic foams currently used in the chemical, pharmaceutical and/orcosmetic industry in environments that do not require high thermalstrength; are easy to prepare, with a low manufacturing cost; and havesimilar or advantageous structural characteristics.

Another aim of the present invention is to provide new catalysts, forheterogeneous and/or supported homogenous catalysis, with a low pressureloss and having a large specific surface, while having very goodchemical inertia, and which are available in all types of geometricshapes (square, planar, cylindrical, etc.) and mechanically flexible.

According to the invention, the problem is resolved by chemicallymodifying the surface of a known cellular material, i.e., a cellularpolymer foam (in particular elastomer) with apparent porosity.

Cellular polymer foams (also called “honeycomb foams”) are well knownand are commercially available for many applications. When they are madeup of closed cells, they have excellent mechanical strength and are usedas foam wedges, packaging foam, or in mechanical construction. At thesame time, these closed cells capture air and thus give the foamsexcellent heat insulation properties, which are used in the buildingsector.

Also known are polymer foams (in particular elastomers) with apparentporosity (called “open-cell polymer foam”), in which only the edges ofthe cells are made up of solid polymer. This in particular involvespolyurethane foams. They are used as filters, in particular inaquariums. However, these foams are not usable as a catalyst substratedue to the low adherence of the catalysts (or their precursor compounds)on the surface of this polymer.

Their use as a catalyst substrate is faced with the difficulty ofdepositing a catalytically active phase or an active phase precursorphase. Indeed, the surface of the polymer bridges (edges) is smooth,with no micro-pores, and does not have a sufficient adherence to depositthe precursor molecules or active phases thereon typically used inhomogenous or heterogeneous catalysis. For this reason, cellular polymerfoams with apparent porosity have hardly been considered as candidatesto manufacture catalyst substrates.

The inventors have found an appropriate surface treatment that preparesthe cellular polymer foams (in particular elastomers), and particularlythose with apparent porosity, to receive a catalytically active phase(here called “active phase”) or active phase precursor phase deposition.

A first object of the present invention is a method for modifying acellular polymer foam with apparent porosity, preferably made frompolyurethane, comprising the following steps: supplying a porouscellular polymer foam with apparent porosity (a) placing said cellularpolymer foam in contact with at least one compound (b) chosen from amongcompounds including at least one catechol unit, and preferably fromamong catecholamines, to obtain a cellular polymer foam comprising, onits surface, an intermediate phase formed from said compound includingat least one catechol unit.

Said cellular polymer foam is advantageously a polyurethane foam.

In one embodiment, the step for placing said cellular polymer foam (a)and the compound (b) in contact is done by immersing said cellularpolymer foam (a) in an aqueous solution of compound (b), or byimpregnating an aqueous solution of compound (b) on said cellularpolymer foam (a), or by partial or complete spraying of an aqueoussolution of compound (b) on said cellular polymer foam (a).

Advantageously, said cellular polymer foam (a) comprises cells with amean size comprised between 500 μm and 5000 μm, preferably between 2000μm and 4500 μm, and still more preferably between 2500 μm and 4500 μm;these values are chosen in order to use the foams as a catalystsubstrate.

Compound (b) preferably includes one (preferably only one) aminefunction, and is advantageously chosen from among catecholamines, and ismore particularly 4-(2-aminoethyl) benzene-1,2-diol (known by the namedopamine), or a derivative thereof. As an example, compound (b) can bechosen from the group made up of: dopamine, noradrenaline, adrenaline,3-methoxytyramine, 4-aminophenol, 3,4-dihydroxyphenyl-L-alanine.

If compound (b) is not an amine, it is preferably chosen from the groupof compounds including at least one catechol unit formed by: caffeicacid, hydroxyhydroquinone, catechol, pyrogallol, morin(2′,3,4′,5′7-pentahydroxyflavone), epigallocatechin, epigallocatechingallate, catechin and its stereoisomers, tannic acid.

The method according to the invention may further include a step c) forfunctionalizing said cellular polymer foam by depositing a phase of atleast one catalytically active material or catalytically active phaseprecursor, said at least one material (c) being selected from the groupmade up of:

metal complexes including at least one group capable of forming covalentbonds with the coating formed from compound (b), i.e., with the catecholor indole structural element (resulting from the cyclization of thealkylamine arm of the catecholamine during step (b)), for example atrialkoxysilane group, an amine group or a thiol group, and moreparticularly the coordinating compounds or organometallic molecules ofthe transition metals;

organic molecules capable of catalyzing the reaction, calledorganocatalysts, including at least one group capable of formingcovalent bonds with the coating formed from compound (b);

metal nanoparticles, preferably metal nanoparticles chosen from amongAg, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Au, Ce, or those of mixed oxidesassociated with these elements, such as Fe2O3, NiO2, Ni2O3, CeO2, aswell as from among those of other oxides, such as TiO2, ZnO, WO3, SnO2,or any possible combinations of these nanoparticles.

This functionalization step can be carried out after depositing compound(b) or simultaneously.

According to one embodiment, the functionalization step is done at atemperature comprised between 5 and 80° C., preferably between 15 and60° C., and still more preferably between 15 and 50° C.

Said metal nanoparticles have a mean particle size comprised between 0.5and 30 nm, preferably between 0.5 and 30 nm, more preferably between 0.5and 20 nm, and still more preferably between 0.5 and 15 nm.

Said catalytically active phase or catalytically active phase precursorcan be deposited using at least one of the following techniques:impregnation, aerosol, or droplets; chemical vapor deposition; capillaryimpregnation.

This step can be carried out after depositing compound (b) orsimultaneously.

The cellular foam implemented in the context of the present inventionassumes the form of blocks (for example, cylindrical or cubic) or plateswith any shape, but the smallest outer dimension must be significantlylarger than the mean size of the cells, and typically at least threetimes this value, preferably at least five times. As a general rule, thesmallest dimension is larger than about 3 mm, preferably larger thanabout 10 mm, and still more preferably larger than about 20 mm.

Another object of the invention is a functionalized cellular polymerfoam with apparent porosity, able to be obtained using the methodaccording to the invention.

Still another object of the invention is the use of a cellular polymerfoam with apparent porosity able to be obtained using the methodaccording to the invention as a catalyst substrate.

Still another object of the invention is the use of a functionalizedcellular polymer foam with apparent porosity able to be obtained usingthe method according to the invention as a catalyst, and moreparticularly as a supported homogenous and/or heterogeneous catalyst.

DRAWINGS

FIGS. 1 to 8 illustrate the invention.

FIG. 1(a) shows a micrograph obtained by optical microscopy of apolyurethane cellular foam with apparent porosity used in the methodaccording to the invention, whereof the cell size is about 4.433±0.362mm; window size of about 2.423±0.341 mm; and bridge diameter of about0.460±0.047 mm. The length of the white bar in the upper right indicates5 mm.

FIG. 1(b) shows a micrograph obtained by optical microscopy of apolyurethane cellular foam with apparent porosity used in the methodaccording to the invention, identifying the characteristic properties ofthe foam (size of the cells “ϕ,” pore sizes (i.e., window size) “a,” andsize of the bridges “ds”).

FIG. 2a shows results obtained by x-ray photoelectron spectroscopy ofthe O1s/C1s atomic ratio, measured on the surface of the polyurethaneelastomer cellular foam of type C31410 Bulpren™ by the companyRecticel®, unmodified and in several states of the method according tothe invention (control C31410 polyurethane foam: diagram; after beingplaced in contact with compound (b) and a single wash=bar A; after twowashes=bar B; after three washes=bar C). An increase is observed in theproportion of oxygen atoms between the control C31410 foam and the foamsmodified by compound (b), control for the adhesion of the polydopamine(here called PDA) (polymerization product of compound (b)). After 1, 2or 3 successive washes, the O1s/C1s ratio remains unchanged, reflectingthe robust nature of the modification.

FIG. 2b shows results obtained by x-ray photoelectron spectroscopy ofthe O1s/C1s atomic ratio, measured on the surface of the polyurethaneelastomer cellular foam of type 8FM2 by Foampartner®, unmodified and inseveral states of the method according to the invention (control 8FM2polyurethane foam: diagram; after placement in contact with compound (b)and a single wash=bar A; after two washes=bar B; after three washes=barC). The increase in the proportion of oxygen atoms relative to carbonatoms between the 8FM2 control foam and the foams modified by compound(b) attests to the grafting of the PDA (compound (b)). This grafting isrobust in light of the lack of variation of the O1s/C1s ratio measuredafter 1, 2 or 3 wash steps; cf. bars A, B and C, respectively.

FIGS. 3a, 3b and 3c show micrographs obtained by scanning electronmicroscopy (SEM) of the polyurethane elastomer cellular foam atdifferent stages of the method according to the invention.

FIG. 3a is a micrograph of the non-modified polyurethane foam of typeC31410 Bulpren™y Recticel®. The length of the white bar in the bottomright indicates 10 μm.

FIG. 3b is a micrograph of the polyurethane foam shown in FIG. 3a afterplacement in contact with an aqueous dopamine solution, also called4-(2-aminoethyl) benzene-1,2-diol (CAS: 51-61-6). The length of thewhite bar in the bottom right indicates 10 μm.

FIG. 3c is a micrograph of the polyurethane foam after the foam obtainedin FIG. 3b is placed in contact with an aqueous solution of TiO2nanoparticles. The length of the white bar in the bottom right indicates1 μm.

FIG. 4 shows the evolution of the absorption spectrum of Acid Orange 7(AO7 or 4-[(2E)-2-(2-oxonaphthalen-1-ylidene-hydrazinyl]sodiumbenzenesulfonate; CAS: 633-96-5) with or without the presence of TiO2nanoparticles and at t=0 and t=22 hours, and at t=22 hours during asecond catalytic cycle with the same reused polyurethane foam.

FIGS. 5a and 5b show micrographs obtained by SEM (acceleration voltage3.0 kV) of an untouched polyurethane elastomer cellular foam (FIG. 5a )and one functionalized by a dopamine deposit (FIG. 5b ) using the methodaccording to the invention. The length of the white bar in the upperright indicates 100 μm.

FIGS. 6a and 6b show micrographs obtained by SEM (acceleration voltage15.0 kV) of a polyurethane elastomer cellular foam afterfunctionalization with dopamine and deposition of a TiO2 nanopowder. Thelength of the white bar in the upper right indicates 1 μm. The darkrectangle in each figure indicates the zone in which the chemicalcomposition has been analyzed by EDX spectroscopy (x-ray emission causedby the electron beam of the SEM apparatus): FIG. 6a shows thecharacteristic emission lines of the elements C, O and Ti, while FIG. 6bonly shows the emission lines of the elements O and C.

FIGS. 7 and 8 relate to experiments done with a catalytic foam preparedaccording to example 8. FIG. 8 shows the stress—deformation curve for anew specimen (curve 1) and after 5,000 compression cycles at 25% (curve2).

FIG. 7 relates to a photocatalytic performance test of a catalytic foamprepared according to example 8. It corresponds to a conversion test asa function of time for a polyurethane elastomer cellular foam afterfunctionalization with dopamine and deposition of a TiO2 nanopowder, new(curve 1) and after 5,000 compression cycles at 25% (curve 2).

DESCRIPTION

The cellular (also called “honeycomb”) polymer foams (and in particularelastomers) used in the context of the method according to the inventionare so-called solid foams with apparent porosity. Preferably, these arepolyurethane foams. The latter are commercially available in largetonnages and at low costs. They are flexible and withstand mechanicaland chemical stresses particularly well, while having morphologicalproperties allowing close mixing of the reagents, and thus theperformance of chemical transformations under gentle conditions(primarily in terms of temperature and pressure) compared to systemswhere the catalyst is deposited on a substrate of a known type notassuming the form of an open-pore foam.

Honeycomb or cellular foams with apparent porosity assume the form ofstructures made up of interconnected cells distributed randomlythroughout the entire structure of the material. At first approximation,cellular foams have a geometry of the regular pentagonal dodecahedrontype, and are classified based on their characteristic sizes (FIG. 1b ):

“ϕ”: mean size of the cells, corresponding to the mean equivalentdiameter of the sphere fitted in the cell;

“a”: mean equivalent diameter of the opening of the pores, also calledsize of the “windows” or size of the “pores”;

“ds”: mean equivalent diameter of the bridges, also calledcharacteristic length of the solid skeleton, here size of the “bridges.”

In the trade, these foams are classified based on the number of pores(windows) per unit of length: PPI (pores per inch).

In another embodiment of the present invention, the modified open-cellfoams according to the invention have a base of synthetic organic foam.This means that the foams according to the invention are manufacturedfrom open-cell foams that include synthetic organic materials,preferably polyurethane foams. The preparation of polyurethane cellularfoams is well known by those skilled in the art. Typically, such a foammay be obtained by polymerization reaction between isocyanate andalcohol.

The method for modifying cellular (or honeycomb) polymer foams (inparticular elastomers) according to the invention is a method formodifying their surface. It comprises (preferably, essentiallycomprises) the following two steps: in a first step, a cellular polymerfoam (a) is provided, preferably a polyurethane elastomer cellular foam,with apparent porosity, in a second step, said cellular polymer foam isplaced in contact with at least one compound (b) chosen from amongcompounds including at least one catechol unit, and preferably fromamong the catecholamines.

Advantageously, said foam includes cells with a mean size ϕ comprisedbetween 500 μm and 5,000 μm, preferably between 2,000 μm and 4,500 μm,and still more preferably between 2,500 μm and 4,500 μm.

Advantageously, the mean equivalent diameter a of the opening of thepores (windows) of the cellular polymer foam is comprised between 100 μmand 5,000 μm, preferably between 800 μm and 3,000 μm. In anotherembodiment, this parameter is between 1,700 82 m and 4,500 μm.

Advantageously, the mean equivalent diameter ds of the bridges of thecellular polymer foam is comprised between 50 μm and 3,000 μm,preferably between 80 μm and 2,500 μm. In another embodiment, thisparameter is situated between 200 μm and 2,500 μm.

Advantageously, the PPI number will be comprised between 5 PPI and 100PPI, preferably between 30 and 75 PPI.

In a first advantageous embodiment, the mean cell size ϕ is between2,000 μm and 4,500 μm, the mean window size a is between 800 μm and3,000 μm, and the mean bridge diameter ds is between 80 μm and 2,000 μm.

In a second advantageous embodiment, the mean cell size ϕ is between2,500 μm and 4,500 μm, the mean window size a is between 1,000 μm and3,000 μm, and the mean bridge diameter ds is between 100 μm and 1,500μm.

In a third advantageous embodiment, the mean cell size ϕ is between3,000 μm and 5,000 μm, the mean window size a is between 1,300 μm and3,500 μm, and mean bridge diameter ds is between 130 μm and 1,750 μm.

“Catecholamine” here refers to a compound including a catechol core(1,2-dihydroxybenzene), the benzene core further including a side chainof alkyl amine, optionally substituted. Components including a singleamine function are preferred over those including several amines or apolyamine (or a component in which a catechol unit reacts with twoamines), which should be avoided. According to the invention, saidcompound chosen from among the catecholamines (which are preferablycatechol monoamines) is advantageously dopamine [4-(2-aminoethyl)benzene-1, 2-diol (CAS: 51-61-6)], or a derivative thereof. Othercompounds having a catechol unit and an amine function that can be usedare for example noradrenaline, adrenaline and3,4-dihydroxyphenyl-L-alanine. However, other molecules simply bearingthe catechol unit may lead to a similar modification of the surface ofthe honeycomb polymer foams.

Thus, as an example, compound (b) can be chosen from the group formedby: dopamine, noradrenaline, adrenaline, 3-methoxytyramine, 4-aminophenol, caffeic acid, hydroxyhydroquinone, catechol, pyrogallol,morin (2′,3,4′,5′7-pentahydroxyflavone), epigallocatechin,epigallocatechin gallate, catechin and its stereoisomers, tannic acid,and 3,4-dihydroxyphenyl-L-alanine.

Although the chemical nature of polydopamine (and a fortiori that of thepolymerization products of the other usable molecules as indicatedabove) is not yet clearly established, the inventors believe that theprimary amine function, present [in] catecholamines, [plays] animportant role in initiating the polymerization process: under oxidizingconditions, the dopamine transforms into an aromatic cycle of the indoletype that is then the real polymerization monomer: the amine present inthis indole derivative is a secondary amine.

Thus, the polymerization process according to the invention leads to acoating that is at least partially polymerized (intermediate phase)having few or no free primary amines; this is important to allow theiruse in catalysis. Indeed, the amine or ammonium surface groups are notfavorable to bind metal or oxide nanoparticles. Groups are preferred ofthe sulfonate, alcohol or alcoholate, carboxylic acid or carboxylic,phenyl or phenolate type. Furthermore, the inventors have found that thepresence of free amine groups does not allow the functionalization offoams including the intermediate phase according to the invention withother molecules of the amine or sulfur derivative type, to further havedegrees of freedom in the modification of their surface for variouscatalytic applications.

For these reasons, in the context of the present invention, compounds ofthe catecholamine type are preferred over compounds including catecholunits without a nitrogen function.

In one embodiment of the method according to the invention, at least onecompound having at least one catechol unit, and preferably a compound inthe catecholamine family, and still more preferably dopamine or4-(2-aminoethyl)benzene-1,2-diol, is supplied with aqueous solution,with a quantity comprised between 0.05% to 10% by weight, preferably0.1% to 1%. The step for placing the elastomer cellular foam (a) and thecompound (b) in contact can be done via several methods. As an example,the contact can be done by immersion of the elastomer cellular foam (a)in an aqueous solution of compound (b); impregnating an aqueous solutionof compound (b) on an elastomer cellular foam (a); or partial orcomplete spraying of an aqueous solution of compound (b) on theelastomer cellular foam (a).

Advantageously, the placement of the cellular polymer foam and theaqueous solution of compound (b) in contact, in particular byimpregnation, immersion or spraying, is done for a period of timecomprised between 1 and 48 hours, preferably 18 and 36 hours, and stillmore preferably between 20 and 30 hours.

Advantageously, the placement of the cellular polymer foam (a) and thecompound (b) in contact is done in a temperature range from 0 to 100°C., preferably from 15 to 35° C., and particularly preferably from 20 to30° C.

In one particularly advantageous embodiment, the cellular polymer foam(a) is placed in contact with an aqueous solution of compound (b) at apH comprised between 5 and 10, preferably between 8 and 9. The pH of thesolution can be checked by using a buffer solution, for example atris(hydroxymethyl)aminomethane hydrochloride buffer solution (Tris·HCl;CAS: 1185-53-1).

Advantageously, the placement of the cellular polyurethane foam (a) andthe compound (b) in contact can be carried out in an aqueous solution orin a water-miscible organic water/solvent mixture. The use of water as asolvent is preferred.

Several avenues for functionalizing cellular polymer foams with amodified surface through the cellular polymer foam transformation methodaccording to the invention were considered and tested, in particular tobe used as a catalyst substrate. To that end, the method according tothe invention comprises an additional step c) for functionalizing thecatalyst substrate by depositing a phase of at least one catalyticallyactive material (c).

In one embodiment of the method for functionalizing the catalystsubstrate according to the invention, the functionalization of thesubstrate is done by grafting a catalyst in the form of a metal complex,in particular a metal complex including at least one group capable offorming covalent bonds with the coating formed from compound (b), i.e.,with the catechol or indole structural element (resulting from thecyclization of the alkylamine arm of the catecholamine during step (b));said group may for example be an alkoxysilane group, a halogenosilanegroup (such as a chlorosilane group), an amine group or a thiol group,and the reactions performing the formation of covalent bonds can forexample be condensation reactions with hydroxyl functions of thecatechol structural element, reactions leading to imine formation,additions of type 1,4. or reactions of the radical type.

Said grafting of a metal complex makes it possible to obtain a supportedhomogenous catalyst also called single-site heterogeneous catalyst.Advantageously, the metal complexes (including at least one groupcapable of forming covalent bonds with the catechol and/or indolestructural element) used in the context of the functionalization methodaccording to the invention are chosen from among organometallicmolecules or coordinating compounds, and more particularly from amongthose comprising at least one transition metal.

In one embodiment of the method for functionalizing a catalyst substrateaccording to the invention, the functionalization of the substrate isdone by grafting a catalyst in the form of an organic molecule(organocatalyst), including at least one group capable of formingcovalent bonds with the coating formed from compound (b), i.e., with thecatechol or indole structural element (resulting from the cyclization ofthe alkylamine arm of the dopamine during step (b)); said group can forexample be an alkoxysilane group, a halogenosilane group (such as achlorosilane group), an amine group or a thiol group, and the reactionsperforming the formation of covalent bonds can for example becondensation reactions with hydroxyl functions of the catecholstructural element, reactions carrying out imine formation, additions oftype 1,4. or radical-type reactions.

The grafting of an organocatalyst makes it possible to obtain asupported organocatalyst.

In one embodiment of the method for functionalizing the catalystsubstrate according to the invention, the functionalization of thesubstrate is done by non-covalent grafting of the catalyst in the formof metal particles, which are advantageously metal nanoparticles. Thegrafting of the metal particles on the surface of the substrate iseffective due to the presence of catechol groups. One thus has asubstrate for heterogeneous catalysts. Advantageously, the metalparticles used in the context of the functionalization method accordingto the invention are chosen from among those of Ag, Fe, Co, Ni, Ru, Rh,Pd, Ir, Pt, Au, Ce, and those of mixed oxides associated with theseelements, for example, Fe2O3, NiO2, Ni2O3, CeO2, as well as from amongthose of other oxides such as TiO2, ZnO, WO3, SnO2, and all possiblecombinations of these nanoparticles. Advantageously, the metalnanoparticles used in the context of the method according to theinvention have a mean particle size comprised between 0.5 and 100 nm,preferably 0.5 and 20 nm, and still more preferably between 0.5 and 15nm. Taking account of the size of the catalytic nano-objects used maydepend on their composition, and in particular their metal or oxidenature: the surface properties are strongly related to the size of themetal nanoparticles (gold and ruthenium, for example), less so in thecase of oxides (TiO2).

Preferably, the step for functionalizing the catalyst substrate is doneby depositing the active phase using at least one of the followingtechniques: impregnation, aerosol, or droplets; chemical vapordeposition; capillary impregnation.

This functionalization step can be carried out after depositing compound(b) or at the same time.

Advantageously, the step for functionalizing the catalyst substrate isdone at a temperature comprised between 5 and 80° C., preferably between15 and 60° C., and still more preferably between 15 and 50° C.

The invention makes it possible to make notable improvements relative tothe heterogeneous catalysts supported by rigid cellular foams, forexample carbon foams, silicon carbide foams, metal foams, or aluminafoams. Indeed, polyurethane foams, which are elastomer foams, have greatflexibility and better impact resistance compared to the metal orceramic foams typically used. Furthermore, the active phases of thecellular foams obtained using the method according to the invention canbe recovered easily, for example through simple combustion of thecellular foams, which is a non-negligible advantage, in particular whenthe materials used as active phase are very costly. For the metal orceramic foams according to the state of the art, recovering the activephase requires complex and polluting chemical treatments.

One particularity of the catalysis substrates according to the inventionis related to the choice of compound (b), which is chosen from amongcompounds having at least one catechol unit, and preferably from amongthe catecholamines. Indeed, the primary drawback of polymer foams foruse as a catalyst substrate is their low temperature resistance: it isnot possible to subject the active phase precursors (for example, metaloxides) deposited on these substrates to the same activation treatments(typically: hydrogen production) as when they are deposited on a metalor ceramic foam because this reaction requires a temperature that risksdamaging the polymer foam. This is one of the main reasons for whichcellular polymer foams with apparent porosity have not been used ascatalyst substrate (the other reason being the low adherence of theactive phase precursors or active phases due to the smooth nature of thesurface of the bridges that form the honeycomb foam). The inventors havefound that modifying the surface of the cellular polymer foams accordingto the invention with a compound having at least one catechol unit, andpreferably chosen from among catecholamines (such as dopamine), makes itpossible to overcome both of these difficulties at the same time. Inparticular, it is possible to do away with the reduction phase of theactive phase precursor deposited on said polymer foam because thecatecholamines act as a reducing agent; this effect is more pronouncedwhen said active phase precursor is deposited in the form ofnanoparticles. Furthermore, the presence of a compound having at leastone catechol unit, in particular the presence of a catecholamine,stabilizes the chemical integrity of the nanoparticles, in particularrelative to the oxidizing action of the air, which may damage ordeactivate the nanoparticles.

Furthermore, when the modified cellular polymer foam that may beobtained using the method according to the invention is used as asupported homogenous catalyst or as a supported organocatalyst (which isimpossible with metal or ceramic foams, which may only be used asheterogeneous metal catalyst substrates), the separation of the productsand the catalyst is made considerably easier relative to thenon-supported catalysts in particular used in fine chemistry or inhydroformylation reactions, which allows the passage to the industrialscale for reactions for which it would have been impossible to considerusing them under homogenous conditions, in particular due to thedifficulty of separating the homogenous catalyst from the reactionproducts. Indeed, in certain sectors of chemistry, such aspharmaceutical chemistry, the residual metal level (coming from a metalcatalyst) in the product obtained by catalysis must not exceed athreshold set at a very low level.

The method according to the invention allows catalytic reactions under“gentler” temperature and pressure conditions than in a traditionalreactor (discontinuous, semi-continuous or closed reactor, called “batchreactor”) owing to the material transfer properties of the foam, inparticular in the case of bi-phase reactions (gas/liquid), as inhydrogenation or hydroformylation.

Thus, the modified cellular foams that may be obtained using the methodaccording to the invention have many advantages relative to the rigidmetal or ceramic foams currently used as catalyst substrates in thechemical, pharmaceutical and/or cosmetic industry. Indeed, rigid metalor ceramic foams have a certain number of limitations. In particular,they lack flexibility: the metal or ceramic foams traditionally usedbreak very easily. Furthermore, the choice of catalyst types that can besupported is limited to metal (nano)particles adsorbed on the surface,therefore only heterogeneous metal catalysts. Furthermore, the catalystsdeposited on these known substrates are not very durable, their agingprimarily being due to the desorption and oxidation of the metal specieson the foam. And lastly, rigid metal or ceramic foams are fairlyexpensive.

The cellular polymer foams modified according to the invention byplacement in contact with at least one compound chosen from among thecatecholamines undergo other chemical functionalization, for examplecovalent grafting of compounds including silane groups, amine groups orthiol groups. This makes it possible to further broaden the spectrum ofsurface properties that can be imparted to the cellular polymer foamsusing the method according to the invention.

EXAMPLES

The invention is illustrated below by eight examples, which arenon-limiting with respect to the invention.

Example 1 pertains to placing a polyurethane cellular foam in contactwith an aqueous dopamine solution.

Example 2 pertains to the functionalization of the cellular foamobtained in example 1 with an aqueous fluoresceinamine solution.

Example 3 pertains to the functionalization of the cellular foamobtained in example 1 with a suspension in aqueous medium of titaniumdioxide TiO2 nanoparticles.

Example 4 pertains to the functionalization of the cellular foamobtained in step 1 with a suspension in aqueous medium of ruthenium(Ru(0)) nanoparticles.

Example 5 pertains to the photocatalysis of acid orange 7 using thefunctionalized cellular foam obtained in example 3.

Example 6 pertains to hydrogenation tests of the styrene using thefunctionalized cellular foam obtained in example 4.

Example 7 pertains to the functionalization of a polyurethane cellularfoam modified by dopamine, by the Michael 1,4-addition of a thiol or theSchiff base reaction (and/or by the Michael 1,4-addition) of an amine.

Example 8 pertains to a functionalization combined into one step of apolyurethane cellular foam with an aqueous dopamine solution containingtitanium dioxide TiO2 nanoparticles.

In these examples, certain abbreviations have been used:

PU: Polyurethane foam

PDA: Polydopamine

PU-PDA: Polyurethane foam comprising an intermediate polydopamine phaseon its surface.

Ru: Ruthenium.

Pu-PDA-Fluo: Polyurethane foam comprising an intermediate polydopaminephase and an upper fluoresceinamine phase.

PU-PDA-Ru(0): Polyurethane foam comprising an intermediate polydopaminephase and a catalytic active phase made up of ruthenium nanoparticles.

PU-PDA-TiO2: Polyurethane foam comprising an intermediate polydopaminephase and a catalytic active phase made up of titanium dioxidenanoparticles.

a.t.: ambient temperature

Example 1: Standard procedure for polydopamine (PDA) grafting on thepolyurethane (PU) foam surface to obtain a PU-PDA foam

In example 1, a polyvalent substrate for heterogeneous catalysts orsupported homogenous catalysts was obtained by placing a polyurethaneelastomer cellular foam in contact with a dopamine solution, this methodcorresponding to reaction diagram 1 below:

In an agitated solution of 4-(2-Aminoethyl)-1,2-benzenediolhydrochloride (CAS: 62-31-7, also called dopamine hydrochloride),prepared by dopamine hydrochloride dissolution (2 mg/mL) in an aqueoussolution (60 mL) of tris(hydroxymethyl)aminomethane (sometimes called“Tris”; CAS 77-86-1) at a molar concentration of 10 mM, the pH of whichis adjusted to 8.5 by the dropwise addition of an aqueous solution ofHCl at 1 M, is submerged in a C31410 Bulpren™ polyurethane foam specimenfrom the company Recticel® or 8FM2 from the company Foampartner®measuring 2×2×2 cm³ and having a mass comprised between 200 and 250 mg.The reaction mixture is agitated at ambient temperature for 24 hours.The dopamine to polydopamine polymerization process is characterized bythe change of color of the reaction medium to dark brown. Thepolyurethane foam grafted with the polydopamine (PU-PDA) is next rinsedwith ultrapure water (MiliQ), then agitated in 50 mL of MiliQ water for10 min. The washing procedure is repeated 5 times. The persistentbrownish color on the surface of the foam is characteristic of theeffective grafting of the PDA. The obtained product is dried bycompressed air flow, then in a drying oven (60-70° C.) for one night.

Analyses of the surface of the foam obtained by x-ray photoelectronspectroscopy (XPS) confirm the adsorption of the polydopamine to thesurface of the C31410 Bulpren™ polyurethane foam specimen from thecompany Recticel® or 8FM2 from the company Foampartner® (cf. FIGS. 2aand 2b ). The significant increase in the O(1s)/C(1s) atomic ratiomeasured by XPS between the non-modified and modified elastomer foamsattests to the presence of PDA and its resistance to washing.Furthermore, scanning electron microscopy (SEM) analyses confirm thepresence of PDA on the surface of the C31410 Bulpren™ polyurethane foamspecimen from the company Recticel® (cf. FIGS. 3a and 3b ).

Example 2: Standard procedure for fluoresceinamine grafting on thesurface of polyurethane foam functionalized with polydopamine (PU-PDA)

In this example, the PU-PDA foam obtained in example 1 wasfunctionalized with an aqueous fluoresceinamine solution to form afluorescent compound called PU-PDA-Fluo. The purpose of this example isin particular to demonstrate that it is possible to functionalize thepolydopamine by reaction with an amine group (by Schiff base reactionand/or Michael 1,4-addition), which makes it possible to consider PU-PDAfunctionalization with any molecule including an amine group, includingmetal complexes. This fluorescent compound in fact allows easy viewingof the functionalization of the polydopamine layer formed on the surfaceof the polyurethane foam using optical fluorescence microscopytechniques.

The PU-PDA foam prepared according to example 1 is submerged in anagitated solution of 5-aminofluorescein (CAS: 3326-34-9, also calledfluoresceinamine, isomer I), prepared by dilution of 5-aminofluorescein(0.5 mg/mL) in an aqueous solution of Tris·HCl at 10 mM (pH 8.5, 60 mL)prepared according to example 1. The reaction mixture is agitated andheated at 60° C. for 3 hours, then agitated at ambient temperature for16 hours. The PU-PDA foam grafted with the fluoresceinamine(PU-PDA-fluoresceinamine) is next rinsed with ultrapure water (MiliQ),then agitated in 50 mL of MiliQ water for 10 min. The washing procedureis repeated 3 times. The PU-PDA-Fluo foam thus obtained is dried bycompressed air flow, then is kept at ambient temperature. Thefluorescence microscopy analysis (excitation at 365 nm) confirms thepresence of fluoresceinamine on the PU-PDA-Fluo foam surface; the layeris continuous and homogenous. The fluorescence intensity (measured usingthe ImageJ® software) on a given surface is twice as high when thePU-PDA foam is modified by fluoresceinamine, relative to the same foamnot modified.

Furthermore, the presence of the fluoresceinamine on the PU-PDA-Fluofoam surface has also been demonstrated by XPS. A decrease of 34% of theO(1s)/C(1s) atomic ratio and of 14% of the N(1s)/C(1s) ratio between thePU-PDA foam and the PU-PDA-Fluo foam attests to the grafting of theamine derivative.

A similar experiment was conducted with the 8FM2 polyurethane foam bythe company Foampartner®, with a similar result.

Example 3: Standard procedure for grafting TiO2 nanoparticles on thepolyurethane foam surface functionalized with polydopamine (PU-PDA)

In this example, the PU-PDA foam obtained in example 1 wasfunctionalized with a suspension in aqueous medium of titanium dioxidemetal nanoparticles to form a catalyst called PU-PDA-TiO2.

One gram of a commercial nanopowder of TiO2 (of the anatase type andwith a particle size of about 15 nm) is added to an aqueous solution ofTris·HCl 10 mM (pH 8.5, 50 mL) prepared according to example 1. Afterhaving been agitated overnight at 1,000 revolutions/minute using amagnetic agitator, the TiO2 suspension is treated by ultrasound for 30min. in a water bath (25° C.) to obtain a well-dispersed suspension ofTiO2 nanoparticles. The PU-PDA foam prepared according to example 1 issubmerged in this suspension of TiO2 nanoparticles, then the reactionmixture is agitated at about 1,000 revolutions/minute and heated at 40°C. for 3 hours. The PU-PDA foam grafted with the TiO2 nanoparticles(PU-PDA-TiO2) is next rinsed with MiliQ ultrapure water, then agitatedin 50 mL of MiliQ water for 10 min. The washing procedure is repeated 5times. The obtained product (PU-PDA-TiO2) is dried by compressed airflow, then in the drying oven (60-70° C.) for one night.

The photoelectron spectrometry analyses (% Ti2p/% C1s=0.09871) andscanning electron microscopy (SEM) analyses confirm the presence of TiO2nanoparticles on the surface of the PU-PDA foam (cf. FIGS. 3c, 6a and 6b).

Example 4: Standard procedure for grafting Ru(0) nanoparticles on thepolyurethane foam surface functionalized with polydopamine (PU-PDA)

In this example, the PU-PDA foam obtained in example 1 wasfunctionalized with a suspension in aqueous medium of metal rutheniumnanoparticles to form a catalyst called PU-PDA-Ru(0). The reactiondiagram for the preparation of this foam is shown below:

Added dropwise to an aqueous solution (50 mL) of RuCl3.3H2O (26.1 mg;0.1 mM) is a freshly prepared aqueous solution (1.8 mL) of NaBh4 at 0.1M under vigorous agitation and at ambient temperature. The reductionprocess occurs quickly and is characterized by the gradual change ofcolor; from dark brown to light brown, then green-brown, and lastly backto dark brown. The addition of NaBH4 is complete when the reactionmedium has become dark brown again (pH<4.9). The colloidal solution isnext agitated for one night at ambient temperature to obtain awell-dispersed suspension of ruthenium(0) nanoparticles, with a particlesize comprised between 1 and 2 nm.

The PU-PDA foam prepared according to example 1 is submerged in thisruthenium nanoparticle suspension, then the reaction medium is agitatedat ambient temperature for 24 hours. The PU-PDA foam grafted with theruthenium(0) nanoparticles (PU-PDA-Ru(0)) is next rinsed with MiliQwater, then agitated in 50 mL of MiliQ water for 10 min. The washingprocedure is repeated 5 times. The PU-PDA-Ru(0) is dried by compressedair flow, then in the drying oven (60-70° C.) for one night.

Example 5: Photocatalysis of acid orange 7 using PU-PDA-TiO2 as catalyst

In this example, a photocatalysis test of acid orange 7 (AO7) was doneusing the PU-PDA-TiO2 cellular foam obtained in example 3. The reactiondiagram is shown below:

Four specimens of PU-PDA-TiO2 foams (previously prepared according toexample 3, and measuring 0.5×2×2 cm3) are submerged in an aqueoussolution (50 mL) of 4-(2-hydroxy-1-naphthylazo)benzenesulfonic acidsodium salt (CAS: 633-96-5, usually called acid orange 7 or AO7) at aconcentration of 2.7.10-5 M (pH˜5.4), placed in a 100-mL beaker. Thesolution is agitated at 250 revolutions/minute using a magnetic agitatorin the dark for 30 min. to balance, then irradiated by 3 bulbs (1visible bulb at 100 W placed above, and 2 UVA bulbs (Amax=365 nm) at 6 Wand 4 W, respectively, placed on the sides). The reaction medium isagitated for 22 hours. The catalytic transformation of the AO7 ischaracterized by the discoloration of the solution and is monitored byUV-visible spectrophotometry (cf. UV spectrums-FIG. 4). This catalyticconversion is complete in 22 hours, and the foams can be reused at leastonce after washing with ultrapure water (MiliQ) without loss ofactivity.

Example 6: Styrene hydrogenation using PU-PDA-Ru(0) as catalyst

In this example, the hydrogenation of the styrene 1 into ethylbenzene 2was done using the PU-PDA-Ru(0) cellular foam obtained in example 4. Thereaction diagram is shown below:

According to the state of the art, this reaction can form as a secondaryproduct of the ethylcyclohexane 3.

The hydrogenation of the styrene 1 is done in a glass 4-neck reactor(250 mL) equipped with a condenser. The PU-PDA-Ru(0) is fixed at the endof the mechanical agitator in a glass cage with holes. The catalyst issubmerged in a styrene solution (0.23 mL; 0.02 M) in ethanol (100 mL).The reaction medium is heated at 70° under mechanical agitation (about450 revolutions/minute) and the hydrogen is boiled continuously in thesolution with the flow rate of 60 mL/min. After 6 and 22 hours ofreaction, 0.5 mL of the reaction medium is withdrawn and analyzed by gaschromatography to determine the conversion. After 6 hours of reaction,4% ethylbenzene 2 is obtained; after 22 hours of reaction, 7%ethylbenzene 2 is obtained. The quantity of ethylcyclohexane 3 formedwas negligible, which emphasizes the selectivity of the reaction withthis catalytic system.

This example also demonstrates that if dopamine is used as compound (b)in the method according to the invention, it is possible to eliminatethe step for reducing the active phase (Ru). This step is necessary oreven essential in the case of systems according to the state of the artwhere the catalyst is deposited on a ceramic, carbon or metal substrate.

Example 7: Functionalization of the PU-PDA foams by grafting a thiol oran amine

HS-(CH2)2-COOH was grafted on a C31410 Bulpren™ polyurethane foam by thecompany Recticel® modified with polydopamine (PU-PDA) via the Michael1,4-addition of the thiol function on the aromatic cycle of the PDA. Tothat end, HS-(CH2)2-COOH was used at a rate of 0.5 mg/mL in an aqueoussolution of NaOH (0.1 M) at ambient temperature. The presence of thethiol on the PU-PDA foam thus modified was demonstrated by XPS. Thesignificant increase in the S(2p)/C(1s) atomic ratio measured by XPSbetween the non-modified PU-PDA foam and the modified PU-PDA foamattests to the presence of the thiol. The same experiment was done withthe 8FM2 polyurethane foam by the company Foampartner® with a similarresult.

In another experiment, NH2-(CH2)6-NH2 was grafted on a C31410 Bulpren™polyurethane foam by the company Recticel® modified with polydopamine(PU-PDA) via a Schiff base reaction (and/or a Michael 1,4 reaction). Tothat end, NH2-(CH2)6-NH2 was used under the following conditions: at arate of 0.5 mg/mL in an aqueous Tris solution (15 mM) with a pH of 8.6at 60° C. for three hours, followed by 16 hours at a.t. The presence ofthe amine on the PU-PDA foam thus modified was demonstrated by XPS. The14% decrease of the O(1s)/C(1s) atomic ratio and the 63% increase of theN(1s)/C(1s) atomic ratio measured by XPS between the non-modified PU-PDAfoam and the modified PU-PA foam attests to the presence of the amine. Asimilar experiment was done with the 8FM2 polyurethane foam by thecompany Foampartner®, with a similar result.

Example 8: Obtainment of a PU-PDA foam grafted with TiO2 nanoparticles(called PDU-PDA-TiO2) in a single step

In this example, a PU-PDA-TiO2 catalyst was obtained in a single step byplacing a polyurethane elastomer cellular foam in contact with asuspension of titanium oxide metal nanoparticles in a dopamine aqueoussolution.

2% (w/v) of a commercial TiO2 nanopowder (of the anatase type and with aparticle size of about 30 nm) was added to an aqueous solution ofTris·HCl 10 mM (pH 8.5) prepared according to example 1. After agitationfor 12 hours at 1,000 revolutions/minute using a magnetic agitator, 2mg/mL of 4-(2-aminoethyl)-1,2-benzenediol hydrochloride (CAS: 62-31-7,also called dopamine hydrochloride) was added. In this TiO2 nanoparticlesuspension, a polyurethane foam specimen similar to that used in example1 is submerged, then the reaction mixture is agitated at about 1,000revolutions/minute and heated at 40° C. for 24 hours. The PU-PDA foamgrafted with the TiO2 nanoparticles (PU-PDA-TiO2) is next rinsed withMiliQ ultrapure water, then agitated in 50 mL of MiliQ water for 10 min.The washing procedure is repeated 5 times. The obtained product(PU-PDA-TiO2) is dried by compressed air flow, then in the drying oven(60-70° C.) for at least 12 hours.

1-17. (canceled)
 18. A functionalized cellular elastomer able to beobtained by a method comprising: supplying a porous cellular polymerfoam with apparent porosity and having a mean equivalent diameter of anopening of the pores comprised between 100 μm and 5,000 μm; and placingsaid porous cellular elastomer foam in contact with at least onecompound including at least one catechol unit, and polymerizing saidcompound including at least one catechol unit on the surface of saidporous cellular elastomer foam, thereby obtaining a substrate comprisingsaid cellular elastomer foam having on its surface an intermediate phaseformed from the polymerisation of said at least one compound includingat least one catechol unit.
 19. The functionalized cellular elastomer ofclaim 18, wherein said compound including at least one catechol unit isa catecholmonoamine.
 20. The functionalized cellular elastomer of claim19, wherein said compound including at least one catechol unit is4-(2-aminoethyl) benzene-1,2-diol or a derivative thereof.
 21. Thefunctionalized cellular elastomer of claim 18, wherein said compoundincluding at least one catechol unit is chosen from the group consistingof: caffeic acid, hydroxyhydroquinone, catechol, pyrogallol, morin(2′,3,4′,5′7-pentahydroxyflavone), epigallocatechin, epigallocatechingallate, catechin and its stereoisomers, tannic acid.
 22. Thefunctionalized cellular elastomer of claim 18, wherein said cellularpolymer foam comprises cells with a mean size comprised between 500 μmand 5,000 μm.
 23. The functionalized cellular elastomer of claim 18,wherein the method further comprises functionalizing said cellularpolymer foam by depositing a phase of at least one catalytically activematerial or at least one catalytically active phase precursor.
 24. Thefunctionalized cellular elastomer of claim 23, wherein thefunctionalized cellular elastomer is a catalyst substrate.
 25. Thefunctionalized cellular elastomer of claim 23, wherein said at least onecatalytically active material is selected from the group consisting of:metal complexes including at least one group capable of forming covalentbonds with the intermediate phase formed from said compound including atleast one catechol unit; organic molecules which are configured tocatalyze a reaction, including at least one group to form covalent bondswith the intermediate phase formed from said compound including at leastone catechol unit; and metal nanoparticles chosen from among Ag, Fe, Co,Ni, Ru, Rh, Pd, Ir, Pt, Au, Ce, or mixed oxides associated with theseelements, or any combination(s) thereof.
 26. The functionalized cellularelastomer of claim 25, wherein the metal nanoparticles have a meanparticle size comprised between 0.5 and 100 nm.
 27. The functionalizedcellular elastomer of claim 18, wherein the functionalized cellularelastomer is mechanically flexible.
 28. A functionalized cellularelastomer comprising: a cellular elastomer foam having an apparentporosity and a mean equivalent diameter of an opening of the porescomprised between 100 μm and 5,000 μm; and an intermediate phase, on thesurface of said cellular elastomer foam, formed from a polymerisation ofat least one compound including at least one catechol unit.
 29. Thefunctionalized cellular elastomer of claim 28, wherein said compoundincluding at least one catechol unit is a catecholmonoamine.
 30. Thefunctionalized cellular elastomer of claim 29, wherein said compoundincluding at least one catechol unit is 4-(2-aminoethyl)benzene-1,2-diol or a derivative thereof.
 31. The functionalizedcellular elastomer of claim 28, wherein said compound including at leastone catechol unit is chosen from the group consisting of caffeic acid,hydroxyhydroquinone, catechol, pyrogallol, morin(2′,3,4′,5′7-pentahydroxyflavone), epigallocatechin, epigallocatechingallate, catechin and its stereoisomers, tannic acid.
 32. Thefunctionalized cellular elastomer of claim 28, wherein said cellularpolymer foam comprises cells with a mean size comprised between 500 μmand 5,000 μm.
 33. The functionalized cellular elastomer of claim 28,further comprising at least one catalytically active material or atleast one catalytically active phase precursor, deposited on the surfaceof the functionalized cellular elastomer.
 34. The functionalizedcellular elastomer of claim 34, wherein said at least one catalyticallyactive material is selected from the group consisting of: metalcomplexes including at least one group capable of forming covalent bondswith the intermediate phase; organic molecules which are configured tocatalyze a reaction, including at least one group to form covalent bondswith the intermediate phase; and metal nanoparticles chosen from amongAg, Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt, Au, Ce, or mixed oxides associatedwith these elements, or any combination(s) thereof.
 35. Thefunctionalized cellular elastomer of claim 34, wherein the metalnanoparticles have a mean particle size comprised between 0.5 and 100nm.
 36. The functionalized cellular elastomer of claim 28, wherein thefunctionalized cellular elastomer is a catalyst substrate.
 37. Thefunctionalized cellular elastomer of claim 28, wherein thefunctionalized cellular elastomer is mechanically flexible.